1. Field of the Invention
This invention relates in general to a classifier, a developer, and an image forming apparatus.
2. Description of Related Art
Copiers, printers, facsimiles and the like, have been known as image forming apparatuses. Some of them employs such an image forming process that a latent image is formed on a latent image carrier using an electrophotographic process, and a powder developing agent (hereinafter, also called toner) is attached to the latent image to develop it as a visible toner image, then the toner image is transferred to a recording medium, including an intermediate transferring member, so that an image is formed.
As a developer developing latent images, which is employed for such image forming apparatuses, such a developer has been known that toner agitated in a developer is carried on the surface of a developing roller, which is a developing agent carrier, and the developing roller is rotated to transfer the toner to the position where the toner is opposite to the surface of a latent image carrier, then the latent image of latent image carrier is developed. After the development, the part of the toner that has not been transferred to the latent image carrier is collected into the developer as the developing roller is rotated, then new toner is agitated, charged and carried on the developing roller to be transferred again.
Another application of an image forming apparatus is also disclosed in the Japanese Laid Open Publication No. 9-197781 and 9-329947. In the disclosed apparatus, a jumping developing method is employed, in which toner is transferred from a developing roller to a latent image carrier without any contact in-between.
Another application of an image forming apparatus is further disclosed in the Japanese Laid Open Publication No. 5-19615. In this apparatus, toner is transferred on the surface of a developing roller by an electrostatic force, and an attractive force generated between the toner and a latent image carrier separates the toner from the surface of the developing roller, thus attaching the toner to the surface of the latent image carrier. Still, another application of an image forming apparatus is disclosed in the Japanese Laid Open Publication No. 59-181375. In this application, toner is transferred to a position where the toner is opposite to a latent image carrier, using a transfer board for transferring the toner by an electrostatic force, then the toner is separated by an attractive force generated between the toner and the latent image carrier and is attached on the surface of latent image carrier.
The Japanese Laid Open Publication No. 7-267363 discloses a powder transfer apparatus for transferring powder, such as toner, using space-traveling-waves fields. This apparatus is provided with electrodes, to which a drive voltage is applied to form space traveling wave fields around the electrodes. The traveling wave fields repel and drive the charged powder, transferring it in the transporting direction of electric fields.
As for a classifier for classifying powder, such as toner, a classifier using a screening or wind force classifying method is known. Other application is disclosed in the Japanese Laid Open Publication No. 8-149859, in which such a classifier is described that classification (separation) is carried out by using the space-traveling-wave fields described above, which make electrostatic force, gravity, and centrifugal force act all together on toner. Besides, the Japanese Laid Open Publication No. 2000-140683 and the Japanese Laid Open Publication No. 2000-140700 disclosed another method such that a voltage is applied to generate a potential vertical to the transfer direction of charged powder so that the powder is separated from the transfer surface according to its specific charge.
To form high quality images using such image forming apparatuses, it is important to keep uniform the quantity of charge and mass of the grains of toner for development. However, it has been found difficult for conventional image forming apparatuses to achieve a uniform attachment of toner. Therefore, in conventional methods, toner is pre-classified in the manufacturing process by screening or applying wind force to uniform the toner to a certain extent, and is supplied to an image forming apparatus.
However, even if the substantially uniformed toner is supplied to the image forming apparatus, the toner is not always uniformly charged because the toner is charged in the image forming apparatus in the first place. Charging the toner in the apparatus inevitably cause the unevenness of q/m (quantity of charge per mass) and of the diameters of the grains of toner, thus posing a problem that there is a limit for forming a high quality image.
Besides, a conventional classifier tends to become a large-sized one. A classifier using a method of electrostatic transfer and gravity to classify toner also has a problem that an exact classification is difficult.